Living free radical initiators based on alkylperoxydiarylborane derivatives and living free radical polymerization process

ABSTRACT

A new class of “living” free radical initiators that are based on alkylperoxydiarylborane and its derivatives and that may be represented by the general formula. 
     
       
         R—[O—O—B-φ 1 (-φ 2 )] n   
       
     
     wherein n is from 1 to 4, R is a hydrogen or a linear, branched or cyclic alkyl radical having a molecular weight from 1 to about 500, and φ 1  and φ 2 , independently, are selected from aryl radicals, based on phenyl or substituted phenyl groups, with the proviso that φ 1  and φ 2  can be the chemically bridged to each other with a linking group or with a direct chemical bond between the two aryl groups to form a cyclic ring structure that includes a boron atom are disclosed. At ambient temperature the R—[O—O—B-φ 1 (-φ 2 )] n  species spontaneously homolyzes to form an alkoxyl radical R—[O*] n , which is active in initiating “living” polymerization of radical polymerizable monomers, and a “dormant” diarylborinate radical *O—B-φ 1 (-φ 2 ), which is too stable to initiate polymerization due to the back-donating of electron density to the empty p-orbital of boron, but which may form a reversible bond with the radical at the growing polymer chain end to prevent undesirable side reactions. The “living” radical polymerization is characterized by a linear increase of polymer molecular weight with monomer conversion, a narrow molecular weight distribution, and the formation of block copolymers by sequential monomer addition.

RELATED APPLICATION

This application is based on Provisional Application No. 60/242,592,filed on Oct. 23, 2000.

FIELD OF INVENTION

The invention relates to a new class of living free radical initiatorsthat are based on alkylperoxydiarylborane derivatives with the generalformula of R—[O—O—B-φ₁(-φ₂)]_(n). The initiators exhibit livingpolymerization at ambient temperature to produce white solid vinylpolymers with pre-determined molecular weight and narrow molecularweight distribution. By sequential monomer addition, the initiators alsoproduce block copolymers with controlled copolymer composition andnarrow molecular weight distribution.

BACKGROUND OF THE INVENTION

The control of polymer structure has been an important facet in polymersynthesis, both for academic interests and industrial applications. Aliving polymerization mechanism provides an optimal means for preparingpolymers having well-defined molecular structures, i.e. molecularweight, narrow molecular weight distribution, polymer chain end, as wellas for preparing block and star polymers. In the past, the most viabletechniques in living polymerization reactions were mediated by anionic,cationic, and recently metathesis initiators [for anionic livingpolymerization, see Holden, et al, U.S. Pat. No. 3,265,765; for cationicliving polymerization, see Kennedy, et al, U.S. Pat. No. 4,946,899; andfor metathesis living polymerization, see R. H. Grubbs, et al,Macromolecules, 21, 1961 (1988)]. However, these polymerizationprocesses are very limited to a narrow range of monomers, due to thesensitivity of active sites to functional (polar) groups.

In many respects, free radical polymerization is the opposite of livingionic and metathesis polymerizations since it is compatible with a widerange of functional groups, but offers little or no control over polymerstructure. Despite this drawback, free radical polymerization is thepreferred industrial choice in the commercial production of vinylpolymers, especially those containing functional groups.

Early attempts to realize a living free radical polymerization involvedthe concept of reversible termination of the growing polymer chains byiniferters, such as N,N-diethyldithiocarbamate derivatives [Otsu, et.al,J. Macromol.Sci., Chem., A21, 961 (1984); Macromolecules, 19, 287(1986); Eur. Polym. J., 25, 643 (1989); Turner, et.al, Macromolecules,23, 1856 (1990)]. However, this strategy suffered from poor control ofpolymerization reaction and polymer formed having high polydispersity.

The first living radical polymerization was observed in the reactionsinvolving a stable nitroxyl radical, such as2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO), that does not react withmonomers but forms a reversible end-capped propagating chain end [see,Moad, et.al, Polymer Bull., 6, 589 (1082); Georges, et.al,Macromolecules, 26, 2987 (1993); Georges, et.al, U.S. Pat. Nos.5,322,912 and 5,401,804; Hawker, et.al, J. Am. Chem. Soc., 116, 11185(1994); and Koster, et.al, U.S. Pat. No. 5,627,248]. The formed covalentbonds reduce the overall concentration of free radical chain ends, whichleads to a lower occurrence of unwanted termination reactions, such ascoupling and disproportionation reactions. For an effectivepolymerization, the reaction has to be carried out at an elevatedtemperature (>100° C.). Relatively high energy is needed in the cleavageof the covalence bond, which maintains a sufficient concentration ofpropagating radicals for monomer insertion. Furthermore, this livingradical polymerization seems effective only with styrenic monomers.

Subsequently, several research groups have replaced the stable nitroxylradical with transition metal species as the capping agents to obtain avariety of copper, nickel, iron, cobalt, or ruthenium-mediated livingfree radical systems, so-called atom transfer radical polymerization(ATRP) [see, Matyjaszewski, et.al, Macromolecules, 28, 7901 (1995); J.Am. Chem. Soc., 117, 5614 (1995); Mardare, et.al, U.S. Pat. No.5,312,871; Sawamoto, et.al, Macromolecules, 28, 1721 (1995); Percec,et.al, Macromolecules, 28, 7970 (1995); Teyssie, et.al, Macromolecules,29, 8576 (1996); and Fryd, et.al, U.S. Pat. No. 5,708,102]. Overall, allof these systems have a central theme, i.e., reversible termination viaequilibrium between active and dormant chain end at an elevatedtemperature, which is regulated by a redox reaction involving metalions. The main advantage of this reaction is that, through a properchoice of the metal compound, it is possible to operate with a broadspectrum of monomers. However, a major drawback is the formation of adeep colored reaction mixture that requires extensive purificationprocedures to obtain the desired final product.

It has also been known that trialkyborane in an oxidized state becomesan initiator for the polymerization of a number of vinyl monomers [seeFurukawa, et al, J. Polymer Sci., 26, 234, 1957; J. Polymer Sci. 28,227, 1958; Makromol. Chem., 40, 13, 1961; Welch, et.al, J. Polymer Sci.61, 243, 1962 and Lo Monaco, et. al. U.S. Pat. No. 3,476,727]. Thepolymerization mechanism involves free radical addition reactions. Theinitiating radicals may be formed from homolysis of peroxyborane or bythe redox reaction of the peroxyborane with unoxidized trialkylborane. Amajor advantage of borane initiators is the ability to initiate thepolymerization at low temperature. Peroxides and azo initiators, whenused alone, usually require considerable heat input to decompose andthereby to generate free radicals. Elevation of the temperature oftencauses significant reduction in molecular weight of the polymeraccompanied by the loss of important properties of the polymer.

U.S. Pat. No. 3,141,862 discloses conducting a trialkylborane-initiatedfree radical polymerization in the presence of an alpha-olefinhydrocarbon polymer. Apparently, the graft-onto reaction by this routewas very difficult. The inert nature and insolubility of polyolefin (dueto crystallinity) also seems to have hindered the process and resultedin very poor graft efficiency. The reactions shown in the examples ofthis patent also seem to require a very high concentration oforganoborane initiator and monomers and to require elevated temperature.The majority products are homopolymers or insoluble gel. No informationabout the molecular structure of copolymers is provided in this patent.

Despite the advantage of borane initiators, organoborane-initiatedpolymerizations tend to be unduly sensitive to the concentration ofoxygen in the polymerization system. Too little or too much oxygenresults in little or no polymerization. High oxygen concentration causesorganoborane to be transfered rapidly to borinates, boronates andborates that are poor initiators at low temperature. Moreover,polymerization is often inhibited by oxygen. To facilitate the formationof free radicals, some borane-containing oligomers and polymers [seeBollinger, et.al. U.S. Pat. No. 4,167,616 and Ritter, et.al. U.S. Pat.No. 4,638,092] were used as initiators in the free radicalpolymerizations. These organoboranes are prepared by the hydroborationof diene monomers or polymers or copolymers. Similar polymericorganoborane adducts, prepared by the hydroboration of 1,4-polybutadieneand 9-borabicyclo(3,3,1)-nonane (9-BBN), have also been reported inMacromol. Chem., 178, 2837, (1977).

In the past decade, we have been focussing on the selective oxidation oftrialkylborane and studying the mono-oxidative adducts as a new freeradical initiation system. The research objective was centered aroundthe functionalization of polyolefins by first incorporating boranegroups into a polymer chain, which was then selectively oxidized byoxygen to form the mono-oxidized borane moieties that initiate freeradical graft-from polymerization at ambient temperature to formpolyolefin graft and block copolymers [Chung, et.al, U.S. Pat. Nos.5,286,800 and 5,401,805; Macromolecules, 26, 3467 (1993); Polymer, 38,1495 (1997); Macromolecules, 31, 5943(1998); J. Am. Chem. Soc., 121,6763 (1999); Macromolecules, 32, 8689(1999)]. Overall, the reactionprocess resembles a transformation reaction from transition metal(metallocene) coordination polymerization to free radical polymerizationvia the incorporated organoborane groups. Several years ago, arelatively stable radical initiator was discovered, i.e., the oxidationadducts of alkyl-9-borabicyclononane (alkyl-9-BBN) [Chung, et.al, J. Am.Chem. Soc., 118, 705(1996)]. This initiator exhibits the radicalpolymerization of methacrylate monomers with a linear relationshipbetween polymer molecular weight and monomer conversion in the range oflow (<15%) monomer conversion. The polymers formed during thepolymerization show a stable but relatively broad molecular weightdistribution (Mw/Mn >2.5), compared to polymers prepared by livingpolymerization processes. This initiator is also incapable of producingblock copolymers by sequential monomer addition, indicating limitedstability at the propagating chain end.

SUMMARY OF THE INVENTION

It is an objective of the present invention to provide a new class ofliving free radical initiators, based on alkylperoxydiarylboranes, whichare pure mono-oxidized borane compounds and can initiate livingpolymerization at ambient temperature to produce vinyl polymers having awhite solid form without the need for performing any purificationprocedures. The general formula of the alkylperoxydiarylboranederivatives is illustrated below:

R—[O—O—B-φ₁(-φ₂)]_(n),

wherein n is from 1 to 4, preferably n is 1 or 2; R is a hydrogen or alinear, branched or cyclic alkyl radical having a molecular weight from1 to about 500, and φ₁ and φ₂, independently, are selected from arylradicals, based on phenyl or substituted phenyl groups, with the provisothat φ₁ and φ₂ can be the chemically bridged to each other with alinking group or with a direct chemical bond between the two aryl groupsto form a cyclic ring structure that includes a boron atom. Thealkylperoxydiarylboranes may be prepared, for example, by (i) theselective oxidation of alkyldiarylborane by oxygen and by (ii) thecondensation reaction between halodiarylborane and alkylhydroperoxide(or the corresponding alkali metal salt).

It is another objective of this invention is to provide a process forpolymerizing vinyl monomers by a “living” free radical polymerizationprocess to prepare vinyl homopolymers and copolymers, including blockand star-shape copolymers, having well-defined molecular structures,i.e., pre-determined molecular weight and narrow molecular weightdistribution. The process involves contacting a mixture comprising oneor more free radical polymerizable monomers with analkylperoxydiarylborane initiator of the present invention at ambienttemperature.

The free radical polymerizable monomers contemplated for use in thisinvention include, for example, methyl methacrylate, ethyl methacrylate,butyl methacrylate, octyl methacylate, methacrylic acid, methylacrylate, ethyl acrylate, butyl acrylate, octyl acrylate, 2-hydroxyethylacrylate, glycidyl acrylate, acrylic acid, maleic anhydride, vinylacetate, acrylonitrile, acrylamide, vinyl chloride, vinyl fluoride,vinylidene difluoride, 1-fluoro-1-chloro-ethylene,1-chloro-2,2-difluoroethylene, chlorotrifluoroethylene,trifluoroethylene, tertrafluoroethylene, hexafluoropropene, styrene,alpha-methyl styrene, substituted styrene, trimethoxyvinylsilane,triethoxyvinylsilane and the like. The radical polymerizable monomersmay be used either singly, or as a combination of two or more differentmonomers.

In the polymerization at ambient temperature, the initiator(R—[O—O—B-φ₁(-φ₂)]_(n)) spontaneously homolyzes at the peroxide bond toform an alkoxyl radical (R—[O*]_(n)) and an diarylborinate radical(*O—B-φ₁(-φ₂)). The alkoxyl radical is active in initiatingpolymerization of the vinyl monomers. On the other hand, thediarylborinate radical is too stable to initiate polymerization due tothe back-donating of electron density to the empty p-orbital of boron.However, this “dormant” borinate radical may form a reversible bond withthe alkoxyl radical at the growing polymer chain end to prevent unwantedtermination reactions. This “living” radical polymerization ischaracterized by a linear increase of polymer molecular weight withmonomer conversion, and by a narrow molecular weight distribution (Mw/Mn<2.0, typically <1.5, preferably <1.2), as well as the production ofblock copolymers by sequential monomer addition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the ¹¹B NMR spectrum of 1-octyl-9-borafluorene;

FIG. 2 illustrates the ¹¹B NMR spectrum of 1-octylperoxy-9-borafluorene;

FIG. 3 illustrates GPC curves of poly(methyl methacrylate) polymersamples prepared in Example 7 (curve (a)), Example 8 (curve (b)),Example 10 (curve (c)), and Example 11 (curve (d));

FIG. 4 is a plot of poly(methyl methacrylate) molecular weight vs.monomer conversion; and

FIG. 5 illustrates GPC curves of (a) poly(methyl methacrylate)(Mn=52,000 g/mol, Mw/Mn=1.2) and (b) poly(methyl methacrylate-b-butylmethacrylate) ) (Mn=122,000 g/mol, Mw/Mn=1.2).

DETAILED DESCRIPTION OF THE INVENTION

The synthesis of well-defined polymers having accurately controlledmolecular structures is an important subject in polymer science due tothe desire to prepare materials with new and/or improved physicalproperties. The issue is particularly important in free radicalpolymerization because it is a preferred industrial process forproducing vinyl polymers. In addition, free radical initiators areuseful with a broad range of vinyl monomers, including monomerscontaining polar groups.

This invention discloses a new class of “living” radical initiators thatcan initiate living radical polymerization at ambient temperature andproduce high molecular weight polymers having well-defined molecularstructures, i.e. pre-determined molecular weight and narrow molecularweight distribution (preferably Mw/Mn <1.2). With a sequential monomeraddition process, the living propagating chain ends (after completingthe polymerization of a first monomer) can cross over to react with asecond monomer to produce a block copolymers. The block copolymersproduced have narrow molecular weight distribution and well-controlledcopolymer composition.

This new class of “living” radical initiators is based on analkylperoxydiarylborane derivative. The general formula is illustratedbelow:

R—[O—O—B-φ₁(-φ₂)]_(n),

wherein n is from 1 to 4, preferably n is 1 or 2; R is a hydrogen (onlyin the case of n=1) or a linear, branched or cyclic alkyl group having amolecular weight from 1 to about 500; and φ₁ and φ₂, independently, areselected from aryl groups.

R groups contemplated for use in this invention include linear andbranched alkyl groups, for example, methyl, ethyl, propyl, butyl, amyl,isoamyl, hexyl, isobutyl, heptyl, octyl, nonyl, decyl, cetyl,2-ethylhexyl, etc., and cyclic alkyl radicals, for example, cyclopentyl,cyclohexyl, cyclooctyl, norbornyl, 1-methylnorbornyl, 5-methylnorbornyl,7-methylnorbornyl, 5,6-dimethylnorbornyl, 5,5,6-trimethylnorbornyl,5-ethylnorbornyl, 5-phenylnorbornyl and 5-benzylnorbornyl. In themultiple-functional initiators (n>1), the R group is employed to connectseveral peroxydiarylborane moieties. The molecular weight of R is below500.

As used in connection with φ₁ and φ₂, the term “aryl group” is meant toinclude C₆ to C₃₀ aryl radicals, such as, for example, phenyl andsubstituted phenyl radicals (C₆H_(5-x)R′_(x)) with one to fivesubstituent groups R′. Each substituent group R′ is, independently, aradical selected from a group consisting of C₁-C₁₅ hydrocarbyl radicals,substituted C₁-C₁₅ hydrocarbyl radicals wherein one or more hydrogenatoms is replaced by a halogen radical, an alkoxy radical, an amidoradical, and a phosphido radical. (C₆H_(5-x)R′_(x)) is also a phenylring in which two adjacent R′—groups are joined forming five toeight-member ring to give a saturated or unsaturated polycyclic phenylgroup such as tetralin, indene, naphthalene, and fluorene. φ₁ and φ₂also can be chemically bridged to each other with a linking group orwith a direct chemical bond between the two aryl groups to form a cyclicring structure that includes a boron atom.

Particularly suitable alkylperoxydiarylborane “living” free radicalinitiators are compounds containing borane moieties, illustrated belowas compounds (I) and (II):

wherein R₁ and R₂ in compounds (I) and (II), independently, represent Hor an alkyl, aryl or alkoxyl substituent having from 1 to 15 carbonatoms, and preferably from 1 to 8 carbon atoms, with R₁ and R₂ beinglocated at all five substitution locations on the respective aryl ringstructures; and wherein X in compound (II) is a linking group selectedfrom a direct chemical bond, —O—, —N(R″)—, —Si(R″₂)— and —(CH₂)_(m)—,where R″ is a C₁-C₄ alkyl group and m is 1, 2 or 3. The preferredinitiators include alkylperoxydimesitylborane,9-alkylperoxy-borafluorene, 1-alkylperoxy-2,3;5,6-dibenzoborine,1-alkylperoxy-2,3;6,7-bibenzoborepin, and1-alkylperoxy-2,3;6,7-bibenzodihydroborepin.

The alkylperoxydiarylborane “living” free radical initiators of thepresent invention may be prepared by (i) a selective oxidation ofalkyldiarylborane by oxygen and by (ii) a condensation reaction betweenhalodiarylborane and alkylhydroperoxide (or the corresponding alkalimetal salt). The following equation illustrates the general reactionscheme for the preparation of an alkylperoxydiarylborane initiatorwithin the scope of the invention, namely:1-octylperoxyl-9-borafluorene:

Detailed preparation procedures for both precursors, i.e.,1-octyl-9-borafluorene (III) and 9-chloroborafluorene (IV), arediscussed in the examples. At ambient temperature, a spontaneousoxidation reaction occurs upon mixing 1-octyl-9-borafluorene (II) withoxygen. Due to the two strong aryl-B bonds, the oxidation reactionselectively takes place at octyl-B bond to produce peroxyborane(C₈—O—O—B) (V). The effective and selective reaction is demonstrated bycomparing the ¹¹B NMR spectrum of a sample of the 1-octyl-9-borafluoreneprecursor (III) before the oxidation reaction (FIG. 1) with the ¹¹B NMRspectrum of a sample of the 1-octylperoxyl-9-borafluorene initiator (V)after the oxidation reaction (FIG. 2). In FIG. 1b, the single chemicalshift of 1-octyl-9-borafluorene (III) at 60 ppm (vs. etherated BF₃)indicates the strong π-electron delocalization around the B atom at thecenter five-member ring, which provides high bond order and goodstability of aryl-B bonds. Upon oxidation, the chemical shift completelymoves to a higher field at 42 ppm (FIG. 2), indicating a quantitativeoxidation reaction to produce 1-octylperoxyl-9-borafluorene initiator(V).

The 1-octylperoxyl-9-borafluorene initiator (V) behaves very differentlyfrom other living radical initiator systems, based on stable nitroxylradical and transition metal species as the capping agents. Thisperoxyborane is a reactive radical initiator even at ambienttemperature. As illustrated in the following equation, the1-octylperoxyl-9-borafluorene initiator undergoes spontaneous hemolyticcleavage of the peroxide moiety to generate a reactive alkoxyl radical(C—O*) (VI) and a stable borinate radical (B—O*) (VII), due to theback-donating of electron density to the empty p-orbital of boron.

The alkoxyl radical (VI) that is formed is very reactive to freeradical-polymerizable monomers, such as methyl methacrylate (MMA) andinitiates radical polymerization at ambient temperature. On the otherhand, the borinate radical (VII) is too stable to react with freeradical polymerizable monomers. However, the borinate radical (VII)serves as the end-capping agent to form a weak and reversible bond withthe growing chain end (VIII) during the polymerization reaction. Uponthe dissociation of an electron pair, the growing chain end (VIII) canthen react with additional monomer, such as methyl methacrylate (MMA),to extend the polymer chain. The resulting new chain end radicalimmediately forms a weak bond with the borinate radical (VII). Such aprocess minimizes undesirable chain transfer reactions and termination(coupling and disproportionation) reactions between two growing chainends. FIG. 3, which illustrates the GPC curves of poly(methylmethacrylate) polymers sampled during the polymerization reactions ofExamples 7, 8, 10 and 11 herein below, clearly demonstrates that polymermolecular weight continuously increases during the entire polymerizationprocess, and that the molecular weight distribution remains quiteconstant and narrow, with Mw/Mn=˜1.2. FIG. 4, which illustrates a plotof polymer molecular weight vs. monomer conversion, shows that all ofthe experimental data points fall almost exactly on the theoreticalline, based on the calculation of [monomer consumed]/[initiatorconcentration]. A near ideal straight line through the origin providesstrong evidence of living polymerization.

Thus, it has been found that the present “living” free radicalinitiators facilitate a “stable” radical polymerization process thatoccurs at ambient temperature. The peroxyborane initiator can beprepared prior to the polymerization or in situ prepared by injection ofoxygen to monomers in the presence of an alkyldiarylborane, such as1-octyl-9-borafluorene. The lack of chain-transfer and termination, bothdisproportionation and coupling reactions, is believed to associatedwith the existence of the “dormant” borinate radical (VII) species,serving as the reversible capping agent, that is produced in situ duringthe formation of initiator.

The free radical polymerizable vinyl monomers contemplated for use inthis invention include, for example, methyl methacrylate, ethylmethacrylate, butyl methacrylate, octyl methacylate, methacrylic acid,methyl acrylate, ethyl acrylate, butyl acrylate, octyl acrylate,2-hydroxyethyl acrylate, glycidyl acrylate, acrylic acid, maleicanhydride, vinyl acetate, acrylonitrile, acrylamide, vinyl chloride,vinyl fluoride, vinylidene difluoride, 1-fluoro-1-chloro-ethylene,1-chloro-2,2-difluoroethylene, chlorotrifluoroethylene,trifluoroethylene, tertrafluoroethylene, hexafluoropropene, styrene,alpha-methyl styrene, substituted styrene, trimethoxyvinylsilane,triethoxyvinylsilane and the like. These radical polymerizable monomerscan be used either singly or as a combination of two or more monomers.The polymers formed from such monomers typically have from about 10 toabout 100,000 repeating monomer units. Preferably the polymers containfrom about 20 to about 30,000, and most preferably from about 50 toabout 10,000 repeating monomer units.

The living radical polymerization of this invention is useful forpreparing homoploymers and random copolymers. However, it is also usefulfor preparing block copolymers by means of sequential monomer addition.In other words, after completing the polymerization of a first monomerto the extent desired to form a first polymer “block”, a second monomeris introduced into the reaction mass to effect polymerization of thesecond monomer to form a second polymer “block” that is attached to theend of the first block. Using this sequential addition process, a broadrange of diblock, triblock, etc. copolymers can be prepared, which havefollowing formula:

where M₁, M₂ and M₃ are independent monomer units chosen from freeradical polymerizable monomers, such as methyl methacrylate, ethylmethacrylate, butyl methacrylate, octyl methacylate, methacrylic acid,methyl acrylate, ethyl acrylate, butyl acrylate, octyl acrylate,2-hydroxyethyl acrylate, glycidyl acrylate, acrylic acid, maleicanhydride, vinyl acetate, acrylonitrile, acrylamide, vinyl chloride,vinyl fluoride, vinylidene difluoride, 1-fluoro-2-chloro-ethylene,1-chloro-2,2-difluoroethylene, chlorotrifluoroethylene,trifluoroethylene, tertrafluoroethylene, hexafluoropropene, styrene,alpha-methyl styrene, substituted styrene, trimethoxyvinylsilane,triethoxyvinylsilane and the like. These radical polymerizable monomerscan be used either singly or as a combination of two or more monomers.The numbers x, y and z represent the number of repeating monomer unitsin each polymer block, and typically x, y and z, independently, would befrom about 10 to about 100,000. Preferably, x, y, and z, independently,would be from about 20 to about 30,000, most preferably from about 50 toabout 10,000. Basically, similar living radical polymerization reactionsoccur sequentially, first with the first monomer (or mixture ofmonomers), then with the second monomer (or mixture of monomers), thenwith the third monomer (or mixture of monomers), and so on, to formdiblock, triblock, etc. copolymers having a narrow molecular weightdistribution, i.e. Mw/Mn <2.0, typically <1.8, and preferably <1.5, andmost preferably <1.2. As shown in FIG. 5, a poly(methylmethacrylate-b-butyl methacrylate) diblock copolymer (graph b) exhibitedalmost twice the molecular weight of a poly(methyl methacrylate)homopolymer (graph a), without changing the narrow molecular weightdistribution. The copolymer composition was basically controlled bymonomer feed ratio.

In accordance with another aspect of the present invention, one of themajor advantages of alkylperoxydiarylborane initiators resides in theability to synthesize initiators containing multiple active sites. Thus,by means of a simple hydroboration reaction of a molecule containingmultiple olefinic double bonds with a diarylborane, followed by thespontaneous and selective oxidation reaction by oxygen at ambienttemperature (as discussed above). The following equation illustrates thepreparation of a difunctional initiatorα,ω-bis(9-peroxyborafluorene)hexane.

The difunctional initiator having two active sites may be used toinitiate a living free radical polymerization to produce high molecularweight polymers within shorter reaction time. It is especially useful ina process for preparing the symmetrical triblock (A—B—A) copolymerscontaining two A end blocks and one B center block. For example, theabove difunctional initiator α,ω-bis(9-peroxyborafluorene)hexane isdissolved in monomer B to form a reaction solution which is polymerizedand forms a telechelic polymer (macroinitiator) containing two activeend groups. The macroinitiator is then contected with monomer A to formsecond reaction solution that is polymerized to form an A—B—A triblockcopolymer.

Furthermore, the chemistry is easily extended to the preparation ofmultiple functional initiators, containing more than two active sites.The reaction involves a compound having multiple olefinicallyunsaturated sites and the same hydroboration reaction of the olefinicsites with a sufficient quantity of diarylborane. The multiplefunctional initiators are very useful for preparing star-shape polymershaving multiple arms, wherein the number of arms is the same as thenumber of active sites in the initiator, and wherein each arm typicallycontains from about 10 to about 100,000 repeating monomer units.Preferably each arm contains from about 20 to about 30,000, and mostpreferably from about 50 to about 10,000 repeating monomer units.

In addition, by using the above-discussed sequential monomer additionprocess, it is also possible to prepare multiple blocks in each arm of astar polymer. In general, the combination of living radicalpolymerization and easy synthesis of multiple functional initiator opensup a wealth of possibilities in the synthesis of complex polymerarchitectures.

The following examples are illustrative of the principles and practiceof this invention.

EXAMPLE 1 Synthesis of 2,2′-Dibromobiphenyl

To a stirred solution of 38.15 g (0.16 mol) of o-dibromobenzene in 350ml of anhydrous tetrahydrofuran (THF) at −80° C. was added 50.5 ml of a1.6 M solution of n-butyllithium in pentane. The rate of addition wassuch that the temperature was not allowed to rise more than 5° C. Afterthe solution was stirred for 2.5 hours at −78° C., the yellow-greenreaction mixture was gradually warmed up to −5° C. over a period of 8hours, and then was hydrolyzed with 100 ml of 5% hydrochloric acid. Theresulting layers were separated and the aqueous layer was extracted withthree 50 ml portions of ether. The ether solution, together with theoriginal organic layer, was dried over anhydrous sodium sulfate, andfiltered. After the removal of the solvent under vacuum, the residue wastreated with 50 ml of anhydrous ethanol and cooled down to 31 40° C. toyield 17.9 g (70% yield) of 2,2′-dibromobiphenyl. One recrystallizationfrom anhydrous ethanol gave pure 2,2′-dibromobiphenyl having a meltingpoint (m.p.) 80.8° C. The molecular structure was determined by ¹H NMRmeasurement in CDCl₃ solvent. ¹H NMR spectra of the pure2,2′-dibromobiphenyl (CDCl₃ at 25° C.) was as follows: δ7.70 (d, 1H,aromatic H), 7.38 (m, 1H, aromatic H), 7.25 (m, 2H, aromatic H).

EXAMPLE 2 Synthesis of 9-Chloroborafluorene

A solution of 5.0 g (16.03 mmol) of 2,2′-dibromobiphenyl in 100 ml ofdiethyl ether was cooled to −10° C. To this solution, a solution of 1.6M n-butyllithium (32.05 mmol) in hexane (21 ml) was added dropwise.After the solution was stirred for 2 hours at −10° C., the yellowreaction mixture was gradually warmed up to −5° C. for 4 hours.Volatiles were then removed under vacuum. The residue was washed withanhydrous hexane, filtered to remove lithium bromide, and dried undervacuum, leaving 2.61 g of dilithiobiphenyl (98% yield) as a colorlesspowder.

To a solution of dilithiobiphenyl (2.61 g, 15.71 mmol) in hexane (100ml) at −10° C. was added dropwise a solution of 1.0 M boron trichloride(15.8 ml) in hexane over a period of 2 hours. The reaction solution wasstirred at −10° C. for 5 hours, and slowly warmed to room temperature,and then stirred overnight. After removal of the solvent under very highvacuum, the product residue was washed with highly pure hexane, filteredand dried under very high vacuum to give 2.18 g (70% yield) of pure9-chloroborafluorene as an yellow powder. The molecular structure wasdetermined by ¹H, ¹³C and ¹¹B NMR measurements in CDCl₃ solvent. ¹H NMRspectra of the pure 9-chloroborafluorene product (CDCl₃, 25° C.) was asfollows: δ7.65 (m, 1H, aromatic H), 7.35 (m, 2H, aromatic H), 7.15 (m,2H, aromatic H). ¹³C NMR spectra of the pure 9-chloroborafluoreneproduct (CDCl₃, 25° C.) was as follows: δ153.52, 135.35, 132.99, 129.31,120.57. ¹¹B NMR spectra of the pure 9-chloroborafluorene product (CDCl₃,25° C.) was as follows: δ63.85.

EXAMPLE 3 Synthesis of Bis(9-Borafluorene)

To a solution of 9-chloroborafluorene (1.93 g, 9.8 mmol) in THF (80 mL)at −78° C. was added dropwise a solution of 1.0 M sodiumtriethylborohydride (9.8 ml) in THF using a syringe over a period of 2hours. The reaction solution was stirred at −78° C. for 2 hours, andslowly warmed to room temperature, and then stirred overnight. After theremoval of the solvent under very high vacuum, the product residue waswashed with high purity THF, filtered and dried under very high vacuumto give 1.12 g (35% yield) of a purified colorless bis-(9-borafluorene)powder. The molecular structure was determined by ¹H, ¹³C and ¹¹B NMRmeasurements in CDCl₃ solvent. ¹H NMR spectra (CDCl₃, 25° C.) was asfollows: δ8.45 (br s, 2H, B—H), 7.42 (m, 1H, aromatic H), 7.21 (m, 2H,aromatic H), 7.02 (m, 2H, aromatic H). ¹³C NMR spectra (CDCl₃, 25° C.)was as follows: δ147.52, 135.15, 132.89, 129.31, 120.19. ¹¹B NMR spectra(C₆D₆, 25° C.) was as follows: δ58.20 (minor species, monomeric B—H),20.12 (major peak, dimer B₂H₂).

EXAMPLE 4 Synthesis of 1-Octyl-9-Borafluorene

To a solution of bis(9-borafluorene) (1.12 g, 3.43 mmol) in rigorouslyanhydrous/anaerobic THF (50 ml) at 50° C. was added dropwise an excessamount of 1-octene (5.0 ml) using a syringe. After stirring for 5 hours,the solvent was removed under very high vacuum. The product residue wasthen washed with highly purity THF, filtered in a highly purified drybox and dried under very high vacuum to give 1.85 g (98% yield) of apurified colorless 1-octyl-9-borafluorene. The molecular structure wasdetermined by ¹H and ¹¹B NMR measurements in CDCl₃ solvent. ¹H NMRspectra of the 1-octyl-9-borafluorene (CDCl₃, 25° C.) was as follows:δ7.49 (m, 1H, aromatic H), 7.30 (m, 2H, aromatic H), 7.12 (m, 2H,aromatic H), 1.69 (tr, 2H, BCH₂), 1.38 (m, 2H, BCH₂CH₂), 1.18 (m, 10H,CH₂), 0.75 (tr, 3H, CH₃). ¹¹B NMR spectra (C₆D₆, 25° C.) was as follows:δ60.17.

EXAMPLE 5 Synthesis of 1-Octylperoxy-9-Borafluorene

To a stirred suspension of 1-octyl-9-borafluorene (1.12 g, 3.43 mmol) inrigorously anhydrous/anaerobic benzene (50 ml) at 20° C., 84 ml ofoxygen (at ambient temperature, 1 atmosphere pressure) was added using asyringe. The mixture was stirred for 3 hours. Evaporation of volatilesunder very high vacuum line afforded 1.0 g (95% yield) of1-octyl-9-borafluorene oxidation adduct as a colorless viscous product.The molecular structure of resulting 1-octylperoxy-9-borafluorene wasdetermined by ¹H, ¹³C and ¹¹B NMR measurements in CDCl₃ solvent. ¹H NMRspectra of the 1-octylperoxy-9-borafluorene product (CDCl₃, 25° C.) wasas follows: δ7.37 (m, 1H, aromatic H), 7.20 (m, 2H, aromatic H), 7.00(m, 2H, aromatic H), 3.45 (m, 2H, OCH₂), 1.39 (m, 2H, OCH₂CH₂), 1.15 (m,10H, CH₂), 0.78 (tr, 3H, CH₃). ¹³C NMR spectra (CDCl₃, 25° C.) was asfollows: δ149.12, 136.25, 132.89, 129.19, 120.57, 69.76, 32.98, 32.42,29.93, 26.16, 22.95, 14.31. ¹¹B NMR spectra (C₆D₆, 25° C.): δ41.94.

EXAMPLE 6 Polymerization of Methyl Methacrylate (MMA)

A 100 ml reactor equipped with a magnetic stirrer was attached to ahigh-vacuum line, and then sealed under a nitrogen atmosphere. Freshanhydrous/anaerobic THF (40 ml) and highly purified MMA (10 ml) wereintroduced via a syringe at 20° C. The reactor was placed in a bath at20° C., and stirred for 10 min. The 1-octylperoxy-9-borafluorene (0.026g) initiator in THF solution was then added, and the mixture was stirredfor 10 hours. The polymerization was quenched with acidic methanol, andthe precipitated polymer (PMMA) was collected, washed, and dried in avacuum oven at 60° C. The conversion of MMA was 17.4%, and the numberaverage molecular weight and molecular weight distribution of theresulting polymer were 18,800 g/mol and 1.21, respectively.

EXAMPLES 7-19 Polymerization of Methyl Methacrylate (MMA)

In a series of examples, following the procedures described in Example6, a 100 mL reactor equipped with a magnetic stirrer was attached to ahigh-vacuum line, and then was sealed under a nitrogen atmosphere. Freshanhydrous/anaerobic THF (40 mL) was introduced via a syringe at 20° C.,followed by adding the amount of MMA indicated in Tables 1 and 2. Thereactor was placed in a bath at 20° C., and stirred for 10 min. Theindicated amount of 1-octylperoxy-9-borafluorene solution in THF wasthen added, and the mixture was stirred for the indicated reaction time.The polymerization was quenched with acidic methanol, and theprecipitated polymer (PMMA) was collected, washed, and dried in a vacuumoven at 60° C. The polymers were characterized by Gel PermeationChromatography (GPC), Differential Scanning Calorimetry (DSC) and NMR.The results for the series of examples are summarized in Tables 1 and 2.

TABLE 1 Polymerization of MMA^(a) by using 1-octylperoxy-9-borafluorene(1.72 mmol/l) Reaction Monomer M_(w)/M_(n) Example Time (hr) Conversion(%) M_(n)(Exp.) M_(n)(Cal.) (PDI) 7 2 2.3  2500  2534 1.17 8 5 5.0  5300 5509 1.19 9 10 17.4 18090 19170 1.21 10 18 33.9 36070 37348 1.18 11 2450.8 54750 55967 1.35 12^(b) 24 54.4 86100 89867 1.46 ^(a)Polymerizationconditions: [MMA] = 1.87M (10 ml), solvent (THF) = 40 ml, ^(b)[MMA] =2.81M (15 ml).

TABLE 2 Polymerization of MMA^(a) by using 1-octylperoxy-9-borafluorene(6.43 mmol/l) Reaction Monomer M_(w)/M_(n) Example Time (hr) Conversion(%) M_(n)(Exp.) M_(n)(Cal.) (PDI) 13 3 9.1  2500  2619 1.14 14 6 18.5 5300  5370 1.19 15 10 35.2 11080 10218 1.21 16 16 56.7 17070 16458 1.1517 20 65.5 20750 19013 1.35 18 28 85.3 25100 24760 1.20 19 28^(b)86.8^(b) 38900 37780 1.25 ^(a)Polymerization conditions: [MMA] = 1.87M(10 ml), solvent (THF) = 40 ml, ^(b)[MMA] = 2.81M (15 ml).

EXAMPLE 20 Polymerization of Butyl Methacrylate (BMA)

A 100 ml reactor equipped with a magnetic stirrer was attached to ahigh-vacuum line, and then sealed under a nitrogen atmosphere. Freshanhydrous/anaerobic THF (40 ml) and highly purified BMA (1.26 M) wereintroduced via a syringe at 20° C. The reactor was placed in a bath at20° C., and stirred for 10 min. 1-octylperoxy-9-borafluorene solution inTHF (1.72 mmol/1) was then added, and the mixture was stirred for 1hour. The polymerization was quenched with acidic methanol, and theprecipitated polymer (PBMA) was collected, washed, and dried in a vacuumoven at 60° C. The conversion of BMA was 1.2%, and the number averagemolecular weight and molecular weight distribution of polymers, asdetermined by GPC, were 1365 g/mol and 1.19, respectively.

EXAMPLES 21-29 Polymerization of Butyl Methacrylate (BMA).

In a series of examples, following the procedures described in Example20, a 100 ml reactor equipped with a magnetic stirrer was attached to ahigh-vacuum line, and then sealed under a nitrogen atmosphere. Freshanhydrous/anaerobic THF (40 ml) was introduced via a syringe at 20° C.,followed by adding the amount of BMA indicated in Table 3. The reactorwas placed in a bath at 20° C., and stirred for 10 min. The indicatedamount of 1-octylperoxy-9-borafluorene solution in THF was then added,and the mixture was stirred for the indicated reaction time. Thepolymerization was quenched with acidic methanol, and the precipitatedpolymer (PBMA) was collected, washed, and dried in a vacuum oven at 60°C. The polymer product obtained in example was characterized by GPC andDSC and NMR. These results are summarized in Table 3.

TABLE 3 Polymerization of butyl methacrylate (BMA) using1-octylperoxy-9-borafluorene Reaction Monomer M_(w)/M_(n) Example Time(hr) Conversion (%) M_(n)(Exp.) M_(n)(Cal.) (PDI) 21 1 1.2  1360  12631.19 22 3 3.0  3500  3153 1.17 23 6 6.7  4870  4966 1.21 24 8 13.2 1411013895 1.18 25 10 19.5 21380 20527 1.18 26 14 29.8 32180 31369 1.41 27 2453.4 57310 56212 1.37 28 36 69.5 74150 73159 1.20 29 48 88.4 95380 930541.22 ^(a)Polymerization conditions: [MMA] = 1.26M (10 ml), solvent (THF)= 40 ml, [1-octylperoxy-9-borafluorene] = 1.72 mmol/l.

EXAMPLE 30 Diblock Polymerization of Methyl Methacrylate and ButylMethacrylate

A 100 ml reactor equipped with a magnetic stirrer was attached to ahigh-vacuum line, and then sealed under a nitrogen atmosphere. Freshanhydrous/anaerobic THF (40 ml) and highly purified MMA (1.86 M) wereintroduced via a syringe at 20° C. The reactor was placed in a bath at20° C., and stirred for 10 min. 1-octylperoxy-9-borafluorene solution inTMF (6.43 mmol/1) was then added, and the mixture was stirred for 10hours. The volatiles, including benzene and MMA monomer, were rapidlyevaporated under high vacuum (below 50 millitorr). A sample of thepolymer (PMMA) was taken from the reactor GPC analysis. A mixture ofbenzene (50 mL) and butyl methacrylate (BMA) (1.26 M) was thenintroduced into the reactor (via syringe) as quickly as possible.Copolymerization was begun at 20° C. and continued for another 10 hours.The copolymerization was quenched with acidic methanol, and theprecipitated polymer PMMA-b-PBMA was collected, washed, and dried in avacuum oven at 60° C. GPC measurements on the sample of the PMMA polymerthat was taken from the reactor showed that the PMMA had a numberaverage molecular weight and a molecular weight distribution of 12,100g/mol and 1.23, respectively. The number average molecular weight andmolecular weight distribution of PMMA-b-PBMA diblock copolymer were20,100 g/mol and 1.35, respectively.

EXAMPLE 31 Diblock Polymerization of Methyl Methacrylate and ButylMethacrylate

A 100 mL reactor equipped with a magnetic stirrer was attached to ahigh-vacuum line, and then sealed under a nitrogen atmosphere. Freshanhydrous/anaerobic THF (40 mL) and highly purified MMA (10 ml) wereintroduced via a syringe at 20° C. The reactor was placed in a bath at20° C, and stirred for 10 min. The 1-octylperoxy-9-borafluoreneinitiator (6.43 mmol) in THF solution was then added, and the mixturewas stirred for 5 h. The volatiles, including benzene and MMA monomer,were rapidly evaporated under high vacuum (below 50 millitorr) and asample of polymer (PMMA) was taken from the reactor for GPC analysis. Amixture of benzene (50 mL) and butyl methacrylate (BMA) (10 ml) wasintroduced into the reactor (via syringe) as quickly as possible.Copolymerization was begun at 20° C. and continued for another 5 h. Thecopolymerization was quenched with acidic methanol, and the precipitatedpolymer (PMMA-b-PBMA) was collected, washed, and dried in a vacuum ovenat 60° C. The number average molecular weight and molecular weightdistribution of PMMA polymer were 5500 g/mol and 1.21, respectively. Thenumber average molecular weight and molecular weight distribution ofPMMA-b-PBMA diblock copolymer were 10200 g/mol and 1.25, respectively.

EXAMPLE 32 Synthesis of α,ω-Bis(9-peroxyborafluorene)octane

To a solution of 9-borafluorene (2.3 g, 7 mmol) in rigorouslyanhydrous/anaerobic THF (100 ml) at 50° C. was added dropwiseα,ω-octadiene (3.5 mmol) using a syringe. After stirring for 5 hours,the solvent was removed under very high vacuum. The product residue wasthen washed with highly purity THF, filtered in a highly purified drybox and dried under very high vacuum to give 2.97 g (97% yield) of apurified colorless α,ω-bis(9-borafluorene)octane. The molecularstructure was determined by ¹H and ¹¹B NMR measurements in CDCl₃solvent.

The oxygen oxidation reaction was carried out by suspending the formedα,ω-bis(9-borafluorene)octane in rigorously anhydrous/anaerobic benzene(100 ml) at 20° C., and adding 320 ml of oxygen (at ambient temperature,1 atmosphere pressure) using a syringe. The mixture was stirred for 3hours. Evaporation of volatiles under very high vacuum line afforded 3.1g (97% yield) of α,ω-bis(9-peroxyborafluorene)octane as a colorlessviscous product. The molecular structure of resultingα,ω-bis(9-peroxyborafluorene)octane was determined by ¹H, ¹³C and ¹¹BNMR measurements in CDCl₃ solvent.

EXAMPLE 33 Triblock Polymerization of PMMA-b-PBMA-b-PMMA

A 200 ml reactor equipped with a magnetic stirrer was attached to ahigh-vacuum line, and then sealed under a nitrogen atmosphere. Benzene(100 ml) and highly purified butyl methacrylate (BMA) (30 ml) wereintroduced via a syringe at 20° C. The reactor was placed in a bath at20° C., and stirred for 10 min. 0.4 g of theα,ω-bis(9-peroxyborafluorene)octane difunctional initiator prepared inExample 32 in THF solution was then added, and the mixture was stirredfor 10 hours. The volatiles, including benzene, THF and the unreactedBMA monomer, were rapidly evaporated under high vacuum (below 10millitorr). A small sample of polymer (PBMA) was taken from the reactorfor GPC analysis and was found to have a molecular weight about 18,500g/mole. A mixture of anhydrous/anaerobic THF (100 mL) and methylmethacrylate (MMA) (15 ml) was introduced into the reactor (via syringe)as quickly as possible. Copolymerization was begun at 20° C. andcontinued for another 10 hours. The copolymerization was quenched withacidic methanol, and the precipitated triblock polymerPMMA-b-PBMA-b-PMMA was collected, washed, and dried in a vacuum oven at60° C. The number average molecular weight and molecular weightdistribution of the triblock polymer were 25,400 g/mol and 1.28,respectively.

What is claimed is:
 1. A living free radical initiator having thegeneral formula. R—[O—O—B-φ₁(-φ₂)]_(n) wherein n is from 1 to 4, R is ahydrogen or a linear, branched or cyclic alkyl radical having amolecular weight from 1 to about 500, and φ₁ and φ₂, independently, areselected from aryl radicals, based on phenyl or substituted phenylgroups, with the proviso that φ₁ and φ₂, can be the chemically bridgedto each other with a linking group or with a direct chemical bondbetween the two aryl groups to form a cyclic ring structure thatincludes a boron atom.
 2. The living free radical initiator according toclaim 1, having the general formula I or II:

wherein, R₁ and R₂ in compounds (I) and (II), independently, represent Hor an alkyl, aryl or alkoxyl substituent having from 1 to 8 carbonatoms, with R₁ and R₂ being located at all available substitutionlocations; and wherein X in compound (II) is selected from a directchemical bond, —O—, —N(R″)—, —Si(R″₂)—, and —(CH₂)_(m)—, with R″ being aC₁-C₂ alkyl group and m being 1, 2 or
 3. 3. The living free radicalinitiator according to claim 1, which comprises analkylperoxydimesitylborane.
 4. The living free radical initiatoraccording to claim 1, which comprises a 9-alkylperoxy-borafluorene. 5.The living free radical initiator according to claim 1, which comprisesa 1-alkylperoxy-2,3;5,6-dibenzoborine.
 6. The living free radicalinitiator according to claim 1, which comprises a1-alkylperoxy-2,3;6,7-bibenzoborepin.
 7. The living free radicalinitiator according to claim 1, which comprises a1-alkylperoxy-2,3;6,7-bibenzodihydroborepin.